The present invention relates to a method and apparatus for inspecting pipes, in particular deep water steel catenary risers (SCR) and gas pipelines (both offshore and onshore, including buried pipelines).
Due to environmental loading, subsea currents and increasing water depths, SCRs are susceptible to fatigue cracking at the touch down areas and at the stress joint locations where they connect to the host platform. The orientation and nature of the fatigue cracks that develop in the circumferential welds and associated heat affected zones are difficult to detect with current in-line or pigging inspection techniques. Frequently an insulating coating is applied to the SCRs and other flowlines to prevent hydrate formation within the pipe. Such coating restricts the use of externally applied inspection techniques.
Critical defects to be detected in SCRs are fatigue cracks in circumferential welds (or in associated heat affected zones) adjacent touchdown areas where the bending stresses are generally greatest and the fatigue life of the SCR is therefore shortest. These critical cracks generally form parallel to the weld and their orientation is therefore circumferential with reference to the pipe.
Corrosion in SCRs and other pipelines is also a major problem in the oil, gas, chemical and other industries. Many pipes are insulated which means that even external corrosion cannot be seen without removing the insulation, which is prohibitively expensive and may lead to damage to the pipe. Internal corrosion and erosion are also particular problems in SCRs and gas pipelines where aggressive fluids are conveyed by the pipelines.
The in-line inspection of gas transmission pipelines is the most appropriate way to check the integrity of the pipeline. The exposure and external inspection of buried pipe on land, is not only extremely costly but in many locations totally impracticable. In offshore locations there may not be the same constraints as those experienced on land to expose the pipeline but in both incidents great care is taken to ensure that the gas transmitted has reached a certain dew point and is dry. It is therefore beneficial that the in-line inspection method does not introduce any moisture into the pipeline during the inspection.
Many of the SCR infield flow lines and risers are between approximately 200 & 305 mm diameter, with manifold pipe work between approximately 100 and 160 mm in diameter. Manifold pipe work, especially on the production side, like the SCRs, normally has an insulating coating in place that can be up to 50.4 mm thick, therefore, like SCRs, external inspection methods can only be used in a limited capacity, for example at designated inspection ports. The smaller diameter lines at the manifold location also make it extremely difficult for the current in-line inspection pigs to pass through tight bends.
Platform caissons can be used for a number of applications offshore, to pump seawater for the platform firewater deluge system, or for discharge purposes. Although their integrity is not as critical as a gas or production riser, their can be serious consequences from their failure. Failures may cause caissons to break off and strike pipelines on the seafloor or lead to failure of the internal pumps that deliver seawater to topside fire fighting systems. The inspection of these components for cracking or wall loss is difficult due to the normally heavy external marine growth cover.
In order to mitigate the risk of failure or leakages it is desirable to inspect and detect fatigue cracking in SCRs and other pipes and flowlines and also to detect any reduction in the wall thickness of such pipes due to corrosion and/or erosion. It is particularly desirable to detect any cracks having a section greater than 1% of the pipe cross section.
Non destructive testing (NDT) techniques for steel pipes, and for inspecting welds in particular, have been developed and such techniques have found application in the inspection of pipelines. Examples of such techniques are X-ray testing, ultrasonic testing, magnetic particle testing, magnetic flux leakage and eddy current testing. However, such known techniques all possess disadvantages that render them unsuitable for use in the inspection of SCRs due to the harsh environment in which SCRs operate and the lack of accessibility to the areas to be inspected.
Magnetic flux leakage (MFL) devices are very common in the oil and gas field, usually provided on pigs for passage through the pipe to be inspected. However, they are not able to reliably detect circumferential cracks and are most suited for detection of corrosion metal loss. Furthermore, MFL devices lack the flexibility to accommodate different pipe diameters. Finally, MFL devices only work on magnetic materials and hence cannot be used for pipes made from a non magnetic stainless steel grade.
Ultrasonic waves can propagate through the wall of a pipe. The presence of defects, such as cracks, in the path of the waves can be detected, either by a detector remote from a source of ultrasonic waves or by detecting reflections from the defect by a detector at or adjacent the source. WO 99/31499 discloses a pig using high frequency (0.5 MHZ to 1 MHz) ultrasonic waves to inspect short lengths of pipe facing the inspection head (typically less than 0.5 m). Such high frequency waves provide high resolution and detection sensitivity but suffer high attenuation and therefore short range. In order to inspect the whole length of a pipe, the pig has to be moved along the pipe while continuously transmitting and detecting ultrasonic waves. Such a system is bulky and expensive and would be unsuitable for many applications, such as gas pipelines, because the pipe needs to be filled with a liquid, such as oil or water, to ensure ultrasonic coupling between the piezoelectric transducers and the pipe wall. Furthermore, such system is very sensitive to contamination on the pipe walls because dirt or trapped air bubbles can cause false defect readings.
Ultrasonic guided waves in the pipe wall, such as Lamb waves, are particularly useful for inspecting pipes for defects because they can be excited at one location on the pipe and will propagate many meters along the pipe, returning echoes indicating the presence of corrosion, fatigue cracks or other pipe defects. Ultrasonic guided waves are guided vibrational modes of a body of material wherein ultrasonic energy is trapped between the boundaries of said body of material and guided through said body of material by the large mismatch in mechanical impedance between the wall of the body of material and a surrounding medium. The terminology is more fully explained in the article by M. G. Silk and K. F. Bainton in the journal “Ultrasonics” of January 1979 at pp 11-19 entitled “The propagation in metal tubing of ultrasonic wave modes equivalent to Lamb waves”.
WO 96/12951 (incorporated herein by reference) discloses the use of long range ultrasonic guided waves to detect faults and reduction in the wall thickness in a pipe. The method relies on the property of low frequency (<100 kHz) guided waves to propagate inside the pipe wall parallel to the pipe axis, with small attenuation, thus making it possible to detect defects located up to tens of meters from the inspection head, said fault detection being made by the detection of the echo reflected by the defect. The method is implemented by dry clamping a ring of piezoelectric transducers to the outer surface of the pipe. However, this technique requires access to the outer surface of the pipe at regular intervals along the length of the pipe. This may not be practical for insulated pipelines, particularly for SCRs which may operate at great depth, making removal of the insulation and subsea depositions on the pipe surface impractical. Such method is also unsuitable for buried pipelines for the same reason.
It is therefore desirable to provide a tool using ultrasonic guided waves that can inspect the pipeline from the inside.